Investments (Amortized Cost and Fair Value of Debt Securities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Securities Available for Sale, Amortized Cost
Due in 1 year or less	$ 265,789
Due after 1 year through 5 years	665,953
Due after 5 years through 10 years	1,540,081
Due after 10 years	56,778
Amortized Cost, Single Maturity Date Securities	2,528,601
Mortgage and asset-backed securities	48,674
Total	2,577,275
Debt Securities Available for Sale, Fair Value
Due in 1 year or less	271,938
Due after 1 year through 5 years	736,333
Due after 5 years through 10 years	1,592,133
Due after 10 years	58,183
Fair Value, Single Maturity Date Securities	2,658,587
Mortgage and asset-backed securities	52,790
Total	2,711,377
Debt Securities Held to Maturity, Amortized Cost
Due in 1 year or less	213,353
Due after 1 year through 5 years	1,181,707
Due after 5 years through 10 years	3,448,906
Due after 10 years	356,995
Amortized Cost, Single Maturity Date Securities	5,200,961
Mortgage and asset-backed securities	1,640,582
Totals	6,841,543	6,510,320
Debt Securities Held to Maturity, Fair Value
Due in 1 year or less	218,435
Due after 1 year through 5 years	1,309,733
Due after 5 years through 10 years	3,562,392
Due after 10 years	379,739
Fair Value, Single Maturity Date Securities	5,470,299
Mortgage and asset-backed securities	1,705,144
Total	$ 7,175,443	$ 6,656,144